Quadrature detection of radio and microwave signals is a powerful and ubiquitous tool used in diverse fields such as nuclear magnetic resonance, MRI imaging, and radar for improving sensitivity and obtaining additional information from the received signals. The traditional approach to measuring both components of a complex signal is to direct the signal to two separate, phase-sensitive detectors having reference inputs that are in quadrature. This is the technique that is commonly used for quadrature detection of radio frequency and microwave signals. However, the use of multiple optical detectors for quadrature detection of laser signals can be undesirable, due to large size, high weight, high power consumption, and/or high cost. Also, the short optical wavelengths of laser signals make it difficult to accurately maintain a quadrature phase relationship between detector references, because variations in temperature and other factors make it virtually impossible to maintain path lengths constant to within a fraction of a wavelength.
Another approach for obtaining quadrature detection of reflected infrared signals, such as are received in LiDAR, is to use a doublet laser excitation pulse, whereby two pulses that are 90 degrees out of phase with each other are transmitted in sequence, whereby the two reflected signals represent the real and imaginary components of the complex response. However, it can be difficult to maintain an accurate quadrature relationship between the two pulses in a doublet laser excitation, due to thermal and other effects. Furthermore, it is often necessary for the two pulses to be spaced very close to each other, so as to avoid artifacts due to movement of detected targets.
Accordingly, the phase shift between the two pulses in quadrature doublet pulse LiDAR excitation must be very fast, as well as highly accurate. Any noise in the phase relationship will significantly limit system performance, and long delays between the pulses will introduce motion artifacts.
What is needed, therefore, is an apparatus and method for rapid, accurate, and stable quadrature phase shifting of doublet laser pulses.